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Substrate specificity and catalytic mechanism of a xyloglucan xyloglucosyl transferase HvXET6 from barley (Hordeum vulgare L.)
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2009 (English)In: The FEBS Journal, ISSN 1742-464X, E-ISSN 1742-4658, Vol. 276, no 2, 437-456 p.Article in journal (Refereed) Published
Abstract [en]

A family 16 glycoside hydrolase, xyloglucan xyloglucosyl transferase (EC, also known as xyloglucan endotransglycosylase (XET), and designated isoenzyme HvXET6, was purified approximately 400-fold from extracts of young barley seedlings. The complete amino acid sequence of HvXET6 was deduced from the nucleotide sequence of a near full-length cDNA, in combination with tryptic peptide mapping. An additional five to six isoforms or post-translationally modified XET enzymes were detected in crude seedling extracts of barley. The HvXET6 isoenzyme was expressed in Pichia pastoris, characterized and compared with the previously purified native HvXET5 isoform. Barley HvXET6 has a similar apparent molecular mass of 33-35 kDa to the previously purified HvXET5 isoenzyme, but the two isoenzymes differ in their isoelectric points, pH optima, kinetic properties and substrate specificities. The HvXET6 isoenzyme catalyses transfer reactions between xyloglucans and soluble cellulosic substrates, using oligo-xyloglucosides as acceptors, but at rates that are significantly different from those observed for HvXET5. No hydrolytic activity could be detected with either isoenzyme. Comparisons of the reaction rates using xyloglucan or hydroxyethyl cellulose as donors and a series of cellodextrins as acceptors indicated that the acceptor site of HvXET can accommodate five glucosyl residues. Molecular modelling supported this conclusion and further confirmed the ability of the enzyme's active site to accommodate xyloglucan and cellulosic substrates. The two HvXETs followed a ping-pong (Bi, Bi) rather than a sequential reaction mechanism.

Place, publisher, year, edition, pages
2009. Vol. 276, no 2, 437-456 p.
Keyword [en]
glycoside hydrolase family 16, molecular modelling, phylogenetic, analyses, plant cell walls, reaction mechanisms, multiple sequence alignment, cell-wall, pichia-pastoris, saccharomyces-cerevisiae, endotransglycosylase, enzyme, oligosaccharides, family, proteins, linkage
URN: urn:nbn:se:kth:diva-18069DOI: 10.1111/j.1742-4658.2008.06791.xISI: 000261978000012OAI: diva2:336115
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-02-09Bibliographically approved
In thesis
1. In vitro and in vivo approaches in the characterization of XTH gene products
Open this publication in new window or tab >>In vitro and in vivo approaches in the characterization of XTH gene products
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]


The xyloglucan endo-transglycosylase/hydrolase (XTH) genes are found in all vascular and some nonvascular plants. The XTH genes encode proteins which comprise a subfamily of glycoside hydrolase (GH) family 16 in the Carbohydrate-Active enZYmes (CAZY) classification. The XTH gene products are believed to play intrinsic role in cell wall modification during growth and development throughout the lifetime of the plant. In the present investigation, biochemical and reverse genetic approaches were used to better understand the functions of individual members of the XTH gene family of two important plants: the model organism Arabidopsis thaliana and the grain crop barley (Hordeum vulgare). A phylogenetic tree of the xyloglucan-active enzymes of GH16 has previously been constructed, where enzymes with similar activities have been shown to cluster together. Several members of phylogenetic Group I/II and III-B, predicted to exhibit xyloglucan endo-transglycosylase activity (EC and members of Group III-A, predicted to exhibit xyloglucan endo-hydrolase activity (EC, were included to analyze the functional diversity of XTH gene products. A heterologous expression system using the yeast Pichia pastoris was found to be effective for recombinant protein production with a success rate of ca. 50%. XTH gene products were obtained in soluble and active forms for subsequent biochemical characterization.

In order to be able to screen larger numbers of protein producing clones, a fast and easy method is required to identify clones expressing active protein in high enough amounts. Thus, a miniaturized XET/XEH assay for high-throughput analysis was developed, which was able to identify activities with good precision and with a reduced time and materials consumption and a reduced work load.

Enzyme kinetic analysis indicated that the XET or XEH activity of all XTH gene products characterized in the present study corresponded to predictions based on the previously revised phylogenetic clustering. To gain insight into the biological function of the predominant XEHs AtXTH31 and AtXTH32, which are highly expressed in rapidly developing tissues, a reverse genetic approach was employed using T-DNA insertion lines of the A. thaliana Columbia ecotype. Genotypic and phenotypic characterization, together with in situ assays of XET and XEH activities, in single- and double-knock-out mutants indicated that these Group III-A enzymes are active in expanding tissues of the A. thaliana roots and hypocotyl.  Although suppression of in muro XEH activity was clearly observed in the double-knock-out, no significant growth phenotype was observed, with the exception that radicle emergence appeared to be faster than in the wild type plants.

Keywords: Arabidopis thaliana, Hordeum vulgare, plant cell wall, xyloglucan, glycoside hydrolase family 16, xyloglucan endo-transglycosylase/hydrolase gene family, xyloglucan endo-transglycosylase, xyloglucan endo-hydrolase, heterologous protein expression, Pichia pastoris, T-DNA insertion, in situ XET/XEH assay, high-throughput screening

Place, publisher, year, edition, pages
Stockholm: KTH, 2011. x, 48 p.
Trita-BIO-Report, ISSN 1654-2312 ; 2011: 1
Arabiodopsis thaliana, Hordeum vulgare, plant cell wall, xyloglucan, glycoside hydrolase family 16, xyloglucan endo-transglycosylase/hydrolase gene family, xyloglucan endo-hydrolase, heterologous protein expression, Pichia pastoris, T-DNA insertion, in situ XET/XEH assay, high-througput screening
National Category
Botany Biochemistry and Molecular Biology Biochemistry and Molecular Biology
Research subject
SRA - Molecular Bioscience
urn:nbn:se:kth:diva-28222 (URN)978-91-7415-848-9 (ISBN)
Public defence
2011-02-02, FB54, Roslagstullsbacken 21, Stockholm, 10:00 (English)
QC 20110114Available from: 2011-01-14 Created: 2011-01-12 Last updated: 2011-01-14Bibliographically approved

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Kaewthai, NomchitEzcurra, InésTeeri, Tuula T.
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